The invention relates to a closed loop hybridising device. In particular, the invention applies to a hybridising device onboard a carrier using information provided by an inertial unit and a satellite navigation system, for example a GPS system.
Carriers such as aircraft or boats have a large number of navigation systems. These navigation means include particularly inertial units, radio-navigation systems using signals output by satellites, and these systems are known as Global Navigation Satellite Systems (G.N.S.S.), baro-altimeters and anemometers.
An inertial unit supplies information with little noise and precise in the short term. However in the long term, positioning performances of an inertial unit degrade more quickly as a function of the quality of sensors and processing used by the unit. Defects in sensors (accelerometers, gyrometers) used in the equipment are largely responsible for this degradation. Information acquired from a satellite navigation system is not very likely to drift in the long term. However, data transmitted by a satellite navigation system has a variable precision and often a great deal of noise
In particularly, hybrid navigation systems use data from a satellite navigation system and an inertial unit and also output relative position and speed information to the carrier. The integrity of hybrid navigation systems is usually monitored using a Kalman filter bank in parallel. These filters hybridise information output by the satellite navigation system and the inertial unit. One of the filters of the bank of filters in parallel denoted as the principal filter, uses all GNSS measurements called pseudo-measurements and information available to it. The other so-called secondary filters of the bank of filters in parallel only use some of the available measurements and information. If a failure occurs at a satellite measurement, the filter that does not receive this measurement will not see it; therefore this filter will remain integral
However, use of this architecture is problematic when it is required to make closed loop hybridising, in other words when it is required to apply corrections calculated by the Kalman filter directly to the inertial calculation device called the inertial unit virtual platform. The presence of a satellite failure then has a direct repercussion on the inertial measurements. The virtual platform is then “polluted” by erroneous corrections due to the use of measurements from a defective satellite to make the corrections. It is also possible to maintain one virtual platform for each Kalman filter, which requires a large number of calculations. However, this problem does not arise for an open loop hybridising in which no correction from the Kalman filter is applied directly onto the virtual platform. But in some cases, for example when the quality of the inertial sensors is not capable of maintaining the inertial errors modelled by the Kalman filter in a linear range, an architecture requiring closed loop hybridising is found to be indispensable.